3,407 research outputs found
Equation of state and transport processes in self--similar spheres
We study the effect of transport processes (diffusion and free--streaming) on
a collapsing spherically symmetric distribution of matter in a self--similar
space--time. A very simple solution shows interesting features when it is
matched with the Vaidya exterior solution. In the mixed case (diffusion and
free--streaming), we find a barotropic equation of state in the stationary
regime. In the diffusion approximation the gravitational potential at the
surface is always constant; if we perturb the stationary state, the system is
very stable, recovering the barotropic equation of state as time progresses. In
the free--streaming case the self--similar evolution is stationary but with a
non--barotropic equation of state.Comment: 9 pages, 2 figure
The Post-Quasistatic Approximation as a test bed for Numerical Relativity
It is shown that observers in the standard ADM 3+1 treatment of matter are
the same as the observers used in the matter treatment of Bondi: they are
comoving and local Minkowskian. Bondi's observers are the basis of the
post--quasitatic approximation (PQSA) to study a contracting distribution of
matter. This correspondence suggests the possibility of using the PQSA as a
test bed for Numerical Relativity. The treatment of matter by the PQSA and its
connection with the ADM 3+1 treatment are presented, for its practical use as a
calibration tool and as a test bed for numerical relativistic hydrodynamic
codes.Comment: 4 pages; to appear as a Brief Report in Physical Review
Electromagnetic radiation produces frame dragging
It is shown that for a generic electrovacuum spacetime, electromagnetic
radiation produces vorticity of worldlines of observers in a Bondi--Sachs
frame. Such an effect (and the ensuing gyroscope precession with respect to the
lattice) which is a reminiscence of generation of vorticity by gravitational
radiation, may be linked to the nonvanishing of components of the Poynting and
the super--Poynting vectors on the planes othogonal to the vorticity vector.
The possible observational relevance of such an effect is commented.Comment: 8 pages RevTex 4-1; updated version to appear in Physical Review
Self-similar and charged spheres in the diffusion approximation
We study spherical, charged and self--similar distributions of matter in the
diffusion approximation. We propose a simple, dynamic but physically meaningful
solution. For such a solution we obtain a model in which the distribution
becomes static and changes to dust. The collapse is halted with damped mass
oscillations about the absolute value of the total charge.Comment: 15 pages, 7 figure
Configurational entropy of black hole quantum cores
Two types of information entropy are studied for the quantum states of a
model for the matter core inside a black hole geometry. A detailed description
is first given of the quantum mechanical picture leading to a spectrum of bound
states for a collapsing ball of dust in general relativity with a non-trivial
ground state. Information entropies are then computed, shedding new light on
the stability of the ground state and the spectrum of higher excited states.Comment: 14 pages, 4 figures, improved versio
Bondian frames to couple matter with radiation
A study is presented for the non linear evolution of a self gravitating
distribution of matter coupled to a massless scalar field. The characteristic
formulation for numerical relativity is used to follow the evolution by a
sequence of light cones open to the future. Bondian frames are used to endow
physical meaning to the matter variables and to the massless scalar field.
Asymptotic approaches to the origin and to infinity are achieved; at the
boundary surface interior and exterior solutions are matched guaranteeing the
Darmois--Lichnerowicz conditions. To show how the scheme works some numerical
models are discussed. We exemplify evolving scalar waves on the following fixed
backgrounds: A) an atmosphere between the boundary surface of an incompressible
mixtured fluid and infinity; B) a polytropic distribution matched to a
Schwarzschild exterior; C) a Schwarzschild- Schwarzschild spacetime. The
conservation of energy, the Newman--Penrose constant preservation and other
expected features are observed.Comment: 20 pages, 6 figures; to appear in General Relativity and Gravitatio
U B V R I Photometry of Stellar Structures throughout the Disk of the Barred Galaxy NGC 3367
We report new detailed surface U, B, V, R, and I photometry of 81 stellar
structures in the disk of the barred galaxy NGC 3367. The images show many
different structures indicating that star formation is going on in the most
part of the disk. NGC 3367 is known to have a very high concentration of
molecular gas distribution in the central regions of the galaxy and bipolar
synchrotron emission from the nucleus with two lobes (at 6 kpc) forming a
triple structure similar to a radio galaxy. We have determined the U, B, V, R,
and I magnitudes and U - B, B - V, U - V, and V - I colors for the central
region (nucleus), a region which includes supernovae 2003 AA, and 79 star
associations throughout NGC 3367. Estimation of ages of star associations is
very difficult due to several factors, among them: filling factor, metallicity,
spatial distribution of each structure and the fact that we estimated the
magnitudes with a circular aperture of 16 pixels in diameter, equivalent to
kpc. However, if the colors derived for NGC 3367 were similar to
the colors expected of star clusters with theoretical evolutionary star tracks
developed for the LMC and had a similar metallicity, NGC 3367 show 51 percent
of the observed structures with age type SWB I (few tens of Myrs), with seven
sources outside the bright surface brightness visible disk of NGC 3367.Comment: Accepted for publication (abr 2007) in The Astronomical Journal (July
2007 issue
Why does gravitational radiation produce vorticity?
We calculate the vorticity of world--lines of observers at rest in a
Bondi--Sachs frame, produced by gravitational radiation, in a general Sachs
metric. We claim that such an effect is related to the super--Poynting vector,
in a similar way as the existence of the electromagnetic Poynting vector is
related to the vorticity in stationary electrovacum spacetimes.Comment: 9 pages; to appear in Classical and Quantum Gravit
Fabry Perot Halpha Observations of the Barred Spiral NGC 3367
We report the gross properties of the velocity field of the barred spiral
galaxy NGC 3367. The following values were found: inclination with respect to
the plane of the sky, i=30 deg; position angle (PA) of receding semi major axis
PA=51 and systemic velocity V(sys)=3032 km/s. Large velocity dispersion are
observed of upt o 120 km/s in the nuclear region, of up to 70 km/s near the
eastern bright sources just beyond the edge of the stellar bar where three
spiral arms seem to start and in the western bright sources at about 10 kpc.
Deviations from normal circular velocities are observed from all the disk but
mainly from the semi circle formed by the string of south western Halpha
sources. An estimate of the dynamical mass is M(dyn)=2x10^11 Msolar.Comment: Accepted to be published in May 2001 issue in the A.J. 19 pages, 7
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